Under certain conditions, some materials can be plastically deformed without rupture well beyond their normal limits, a property called superplasticity. This property is exhibited by certain metals and alloys, within limited ranges of temperature and strain rate. For example, titanium and its alloys are superplastic in the temperature range 1450.degree.-1850.degree. F.
Superplastic forming (SPF) is a fabrication technique that relies on superplasticity. A typical SPF process involves placing a sheet of metal in a die, heating the sheet to an elevated temperature at which it exhibits superplasticity, and then using a gas to apply pressure to one side of the sheet. The pressure stretches the sheet and causes it to assume the shape of the die surface. The pressure is selected to strain the material at a strain rate that is within its superplasticity range at the elevated temperature.
One advantage of SPF is that very complex shapes can be readily formed. In addition, the SPF process is generally applicable to single and multi-sheet fabrication, and can be combined with joining processes such as diffusion bonding to produce complex sandwich structures at a relatively low cost. The simplicity of the SPF process leads to lighter and less expensive parts with fewer fasteners, and higher potential geometric complexity. Common applications of SPF include the manufacturing of parts for aircraft, missiles and space vehicles.
In a typical prior art SPF process for titanium, a titanium sheet is placed between steel dies, one of which has a contoured surface corresponding to the shape to be imparted to the titanium sheet. The dies are then placed on platens or plates which are heated through the use of electrical resistance type heating elements embedded within the platens. The platens heat the dies through conduction heating to about 1650.degree. F. To avoid oxidation of the titanium at the elevated temperature, the sheet is immersed in an inert atmosphere such as argon gas. The dies conduct heat into the titanium until its temperature reaches the superplastic range. At that time, the pressure of the argon gas on the side of the sheet away from the contoured surface is elevated sufficiently to deform the titanium sheet against the contoured surface, whereupon the sheet acquires the shape of the surface.
The high temperature at which the SPF operation must be carried out causes it to be a slow and cumbersome process. In particular, because of their large thermal mass, the dies are typically maintained at forming temperature throughout a production run. Failure to maintain the dies at superplastic forming temperatures during part loading and unloading would result in unacceptable process times for each part. Thus, blank sheets must be inserted, and formed parts removed, while the SPF dies are at forming temperature. Because the parts are loaded and unloaded from the dies while still at forming temperature, the parts must be very carefully handled in order to minimize bending of the part. Even with careful handling, some parts may be distorted during unloading and require subsequent processing steps to achieve proper part tolerances. Furthermore, the elevated temperature of the forming dies and parts requires operators to wear protective clothing and use special equipment to insert the metal sheets between the dies, and particularly to remove the formed parts.